Collagen is the most abundant material in the extracellular matrix (ECM) of the human body that surrounds cells and forms the cell-interactive scaffolding of the body. As such, it is completely biocompatible. Hence, it is an excellent biomaterial but has poor mechanical and poor enzymatic stability. In addition, due to limited functionalities (amine and acid) collagen does not provide the flexibility to create a series of multi-functional matrices that are necessary for use as “designer” biomaterials. Therefore, new methodology for modification of collagen with new functional groups, which can lead to diverse chemistry to fabricate covalently linked multi-component biomaterials for regenerative medicine is desirable.
For many tissue engineering and bio-medical applications there is a need for chemically crosslinked collagen materials. A variety of cross-linking procedures are described in the literature. For example to increase its mechanical and enzymatic stability, collagen molecules can be covalently cross-linked leading to the formation of a stable hydrogel. Chemically crosslinked collagen via N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) coupling with zero-length crosslinking has been successfully transplanted into humans as an artificial cornea. However, in cases of severe disease condition or microbial attack, where higher secretions of collagenase and matrix metalloproteinase (MMP) enzymes occur, the mechanical stability of collagen-based artificial corneas needs to be enhanced to reduce enzymatic degradation.
Additionally, reproducibility in the preparation of collagen hydrogels via EDC/NHS coupling can be an issue, resulting in batch-to-batch variability. EDC is also highly susceptible to hydrolysis, making this crosslinking procedure unsuitable for dilute collagen concentrations, for instance for encapsulating cells in a collagenous 3-D matrix. Cell encapsulation cannot be carried out with EDC cross-linked collagen due mainly to the smaller pore size of the resulting hydrogel, which (1) impedes nutrient flow to the cells, and (2) squeezes the cells themselves beyond their physiological limit. EDC/NHS coupling reaction also liberates isourea as by-product, which is an established toxic compound.
All the above-mentioned limitations (variations in the hydrogel degree of crosslinking and impossibility in using EDC/NHS coupling for cell encapsulation purposes) constitute an obstacle for its implementation in cornea applications. Different alternatives have already been investigated to improve artificial cornea stability by keeping its fundamental properties constant, like transparency, cell adhesion and hydration. However, there is a general need in enhancing hydrogel mechanical properties in terms of percentage of deformation and elasticity. These issues are further enhanced during suturing of the collagen hydrogels in surgery (cornea transplantations), where brittle hydrogels are an obvious concern.
3D printing is one of the latest “hot” technologies being developed. The most flexible 3D printers extrude “ink” through a syringe positioned and operated by computer-controlled motors, allowing a wide range of materials and stem cells to be used. Biopolymers, hydrogels, and bio-compatible polymers are relatively easy to fabricate with this method, but a wide range of other functional materials such as conducting polymers and slurries of hard materials can also be used. Syringe-based printers can be designed with multiple-syringes, or the material in the syringe can be replaced during printing, so that relatively complex assemblies of “living” and otherwise “functional” (for example, electronically-conducting) material can be combined in the same device. 3D printing of organs is a fascinating new area but will still requires the development of appropriate “inks” that are biologically and clinically relevant and that will allow encapsulation of living cells, e.g. stem cells.
Assembly of multiple components that have been pre-fabricated using different methods is yet another method for fabrication of a multi-tissue organ. However, as with 3D printing, there is a need for multicomponent, biointeractive materials.
Even though many hydrogels possess the biocompatibility properties needed they lack in light transmission, i.e. they are not transparent enough.